In recent years, improvements in liquid crystal display device performances have become inevitable with progresses of the information-intensive society. Liquid crystalline compositions have been required to have a large refractive index anisotropy in order to improve the speed or performances of liquid crystal display devices.
Known low molecular weight liquid crystalline materials having a large refractive index anisotropy include compounds having a Schiff base or pyrimidine structure (Non-Patent Literature 1), tolan-based compounds (Patent Literatures 1 and 2), and diacetylene-based compounds (Non-Patent Literature 2, and Patent Literatures 3, 4, 5, and 6). The compounds described in these patent literatures are those with the (di)acetylene backbone having at the both ends a phenylene or naphthylene group to which an alkyl or alkoxy group bond and are described to be used as liquid crystal materials to improve the response performance of a liquid crystal display device utilizing their large refractive index anisotropy. However, although these documents disclose that the compounds have a sufficiently large refractive index anisotropy, they do not indicate the specific numeric values thereof or do not describe about the stability to heat or light.
Meanwhile, high molecular weight liquid crystalline compounds (polymers), which can be easily aligned and fixed in a liquid crystal state have been used in optical elements such as compensators or polarizers for liquid crystal display devices after being fixed in an aligned state. However, a more highly functional liquid crystalline polymer has been sought because in connection with liquid crystal display devices which have been thinner and enhanced in functions, constituting components of the devices has also been required to be enhanced in functions.
Known liquid crystalline polymers with a large refractive index anisotropy include polymers of (meth)acrylate having a tolan (acetylene) structure (Patent Literature 7). Polysiloxanes having a diacetylene backbone at a side chain and exhibiting liquid crystallinity (Non-Patent Literature 3) are also known, but they have a high melting point and thus cannot be sufficient in forming processability.